Progressive multifocal leukoencephalopathy (PML) is an infection of the white matter of the brain caused by the JC virus. There is no known effective treatment for PML, but there are case reports of improvement with antiretroviral medications, with intravenous cytosine arabinoside and with intrathecal Ara-C. We are planning a multicenter study through the AIDS Clinical Trials Group to evaluate these three therapies.